A. Field of the Invention
The present invention relates generally to photonic crystals, and, more particularly to a system for efficient coupling to photonic crystal waveguides.
B. Description of the Related Art
During the last decade photonic crystals (also known as photonic band gap or PBG materials) have risen from a relatively obscure technology to a prominent field of research. In large part this is due to their unique ability to control, or redirect, the propagation of light. E. Yablonovich, xe2x80x9cInhibited spontaneous emission in solid-state physics and electronics,xe2x80x9d Physical Review Letters, vol. 58, pp. 2059-2062 (May 1987), and S. John, xe2x80x9cStrong localization of photons in certain disordered dielectric superlattices,xe2x80x9d Physical Review Letters, vol. 58, pp. 2486-2489 (Jun. 1987), initially proposed the idea that a periodic dielectric structure can possess the property of a band gap for certain frequencies in the electromagnetic spectra, in much the same way as an electronic band gap exists in semiconductor materials. This property affords photonic crystals with a unique ability to guide and filter light as it propagates through it. In this way, photonic crystals have been used to improve the overall performance of many opto-electronic devices.
The concept of a photonic band gap material is as follows. In direct conceptual analogy to an electronic band gap in a semiconductor material, which excludes electrical carriers having stationary energy states within the band gap, a photonic band gap in a dielectric medium excludes stationary photonic energy states (i.e., electromagnetic radiation having some discrete wavelength or range of wavelengths) within a certain energy range or corresponding frequency range. In semiconductors, the electronic band gap results as a consequence of having a periodic atomic structure upon which the quantum mechanical behavior of the electrons in the material give rise to a forbidden range of energy levels, the so called electronic band gap. By analogy, the photonic band gap results if one has a periodically structured material, where the periodicity is of a distance suitable to interact with an electromagnetic wave of some characteristic wavelength, in such a way as to create a band of frequencies that are forbidden to exist within the material, the so called photonic band gap.
An envisioned use of these materials is the optical analog to semiconductor behavior, in which a photonic band gap material, or a plurality of such materials acting in concert, can be made to interact with and control light wave propagation in a manner analogous to the way that semiconductor materials can be made to interact with and control the flow of electrically charged particles, i.e., electricity, in both analog and digital electronic applications.
Planar photonic crystal circuits such as splitters, high Q-microcavities, and multi-channel drop/add filters have been investigated both theoretically and experimentally in both two- and three-dimensional photonic crystal structures. For two-dimensional photonic crystal structures, the photonic crystal consists of either an array of low index cylinders surrounded by a background material of sufficiently higher index or, an array of high index cylinders surrounded by a background material of sufficiently lower index. In both cases, in-plane confinement is achieved through multiple Bragg reflections that occur due to the presence of the material lattice, which represents the photonic crystal. For some three-dimensional photonic crystal structures, namely those that consist of a two-dimensional structure, or lattice, that are finite in height, confinement in the vertical direction is achieved through total internal reflection (TIR). In either case the main limiting factor in the wide spread use of these devices is the ability to get light into and out of these structures. For this reason, optical coupling structures have a pronounced impact on the operation of any photonic integrated circuit (xe2x80x9cPICxe2x80x9d).
There have been many types of coupling structures presented in the literature that include, among others, grating couplers and focusing grating couplers. These devices have been used to achieve coherent light coupling both into and out of a waveguide. Grating couplers have a periodicity in a single spatial direction, whereas focusing grating couplers (or grating lenses) have a curvilinear grating.
Unfortunately, coupling to photonic crystal structures has limited the true integration and implementation of photonic crystal integrated circuits. Photonic crystal waveguides have dimensions in sub-wavelength scale, which is favorable for device dimensions, but such small dimensions make efficient couple to photonic crystal structures exceedingly difficult.
While numerous attempts have been made to overcome the challenge of photonic crystal coupling, including tapered couplers, but coupling, coupled waveguides, and prism coupling, such attempts represent, more or less, variations of conventional techniques that have been used to couple optical signals to traditional dielectric waveguides. While these conventional techniques are also capable of coupling to photonic crystal waveguides, they do not provide highly efficient coupling to and from these waveguides, and they are highly susceptible to profile variations that are introduced during fabrication (e.g., errors introduced during the fabrication process). Such fabrication variations change and may completely diminish the coupling efficiency.
Thus, there is a need in the art for a coupling technique for photonic crystal structures that is not sensitive to fabrication variations, and is easy to fabricate and replicate using existing technology. Efficient coupling to and from photonic crystal circuits will open many new exciting opportunities in integrated optics and high-density optical interconnection, and provide true realization of a photonic crystal integrated circuit (xe2x80x9cPCICxe2x80x9d).
The present invention solves the problems of the related art by providing coupling to photonic crystal circuits using a reflective structure, such as a dielectric mirror, that efficiently couples an optical signal to and from photonic crystal waveguides.
As embodied and broadly described herein, the present invention is broadly drawn to an optical coupling system, comprising: a photonic crystal structure having one or more waveguides provided therein; a dielectric waveguide through which an optical signal is provided; and an optical coupler having a mirror, the optical coupler optically couple the optical signal from said dielectric waveguide to the one or more waveguides provided in said photonic crystal structure.
As further embodied and broadly described herein, the present invention is drawn broadly to a method of forming an optical coupling system, comprising: forming a photonic crystal structure having one or more waveguides provided therein, the photonic crystal structure being formed on a substrate; providing an optical signal source; and forming an optical coupler having a mirror on the substrate, the optical coupler optically couple an optical signal from the optical signal source to the one or more waveguides provided in the photonic crystal structure.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.